Drying and gasification process

ABSTRACT

A process for producing syngas from a carbonaceous substance and/or treating a carbonaceous substance, the process including the following steps: a) reducing the surface moisture of the carbonaceous substance; b) reducing the inherent moisture of the carbonaceous substance; and, c) gasifying the carbonaceous substance to produce syngas, wherein at step a) the carbonaceous substance is directly contacted with a hot gas at a temperature of between 50° C. to 250° C., and/or the carbonaceous substance is indirectly contacted with saturated steam at a temperature of between 105° C. and 250° C.

The present invention relates to a process for drying and subsequentlygasifying a carbonaceous substance and in particular a process usingcarbonaceous substances that have high moisture content.

BACKGROUND OF THE INVENTION

In recent times, reducing CO₂ emissions has become increasinglyimportant on a global scale, particularly in relation to the productionof the world's electricity supply which at present relies heavily oncoal fired power stations. Biomass is a renewable energy source that canprovide a genuine alternative to coal in that it can be used for baseload electricity generation. However, biomass has failed to make a widespread difference to the local or global energy market for two reasons:

-   -   1. Biomass typically has a very high moisture content of 30 to        80%. This usually results in a larger and more expensive boiler        plant and lower conversion efficiency of thermal to electrical        energy.    -   2. Biomass is distributed across large areas of land, making        collection and transport to a central power station        prohibitively expensive.

Nevertheless, inefficient open cycle power generation is widelypracticed in Queensland using sugar cane bagasse and in Scandinaviausing wood waste. These open cycle power plants tend to be over 5 MW_(e)in size as steam turbines do not scale down in size economically. Inareas where biomass availability is less, which is most of the world,5MW_(e) is too big for a viable biomass power station that uses opencycle power generation.

During World War 2 biomass gasifiers were commonly used to power fourstroke engines. For example; char producer gas (syngas) systems, usingwood charcoal, and air blown down draft gasifiers, powered many carsduring the petrol rationing. This approach has some merit asreciprocating internal combustion engines economically scale down insize and there is maintenance support for these engines in ruralcommunities. Rural communities also tend to have a higher biomassdensity per unit area. Unfortunately, the down draft char gas systemdoes not accept a wide range of biomass and the wood or char that theydo accept must be carefully sized and dry.

Fluidised bed gasifiers accept a large range of biomasses and have awide size distribution but are traditionally unacceptable for use withreciprocating internal combustion engines due to excessive tarcarryover.

Some biomasses, such as manure are relatively concentrated at poultrysheds and feed lots. Unfortunately these biomasses emit ammonia attemperatures near 100° C. and sulphurous compounds, like H₂S and COS,between 150 and 300° C. In addition, the waste from poultry sheds oftenincludes dead chickens which could also be used as a biomass fuelsource, however these give off hydrogen cyanide when heated totemperatures around 300° C.

The trend to back-plant Australia's sheep and wheat country with 100metre wide strips of Australian natives, like sugar gums and malleetrees, has been encouraged by some farming co-operatives. This backplanting helps the wheat field from being dried by wind and providesshelter for the sheep. This has been encouraged to diversify farmer'sincome with timber from the sugar gums and eucalyptus oil from themallee tree. A similar trend of back planting farm land is apparent inthe USA with strong consideration of cropping the woody switch grass andin Scandinavia the hybrid willow-poplar.

These initiatives will increase the amount of biomass in rural areas andmake them more drought proof. Unfortunately, the biomass from theseactivities tends to be of low quality due to very high moisture contentcompared to wood. This is as a result of the high proportion of leavesand boiled up mash used to recover eucalyptus and other essential oils.Tea tree oil biomass has similar problems.

There have been several attempts at increasing the efficiency of lowrank coal power generation and reducing its subsequent CO₂ emissionssuch that it compares to electricity plants using so called higher rankcoals.

The difficulty with low rank coals, such as brown and sub-bituminouscoals is their high moisture content which typically results in a largerand more expensive boiler plant and lower conversion efficiencies.

One process that has attempted to increase efficiency and reduce CO₂emissions of brown coal power generation is the IDGCC process(integrated drying gasification combined-cycle). IDGCC uses an air blownfluidised bed gasifier to convert brown coal to fuel gas. The IDGCCprocess initially dries the brown coal to remove surface moisturecontent prior to feeding it into a gasifier. The integrated dryingconcept removes the surface moisture of the raw coal under pressure bydirect contact with the hot gas leaving the gasifier. The dried coalthen goes directly to the gasifier and the cooled and humidified gas iscleaned and sent to a gas turbine combined-cycle plant. Using air as thegasifying agent, the calorific value of the gas is very low, but it isacceptable for combustion in a gas turbine. By integrating the coaldrying and gas cooling, substantial cost savings are made with the IDGCCprocess whilst achieving high efficiencies and low CO₂ emissions throughthe combined cycle.

In the IDGCC process the gasifier operates at a temperature of over 950°C. with air plus some steam as the gasifying agent. The hot gas leavingthe gasifier at the top passes through a cyclone that returns most ofthe carry-over dust back to the gasifier. The gas is subsequently burntin a gas turbine.

The main disadvantage with HRL's IDGCC process, with respect to biomass,is the cost associated with building a vertical fluidised bed gasifierthat can operate at elevated temperatures above 950° C. and at pressuresexceeding 10 atmospheres and doing so for a plant as small as 0.5 to 5MW_(e). The HRL design cannot separate ammonia and sulphurous compoundsfrom the exhaust gas and all these undesirable gases pass through to theturbine. The HRL design would have minor tar carryover which is not aproblem with a gas turbine as the gas is burnt outside the turbine.However, it is a problem with gas engines where the gas is burnt insidethe engine.

A Steam Fluidised Bed Drying concept developed at Monash Universityinvolved drying the coal in a superheated steam fluidised bed with theproduct water vapour recompressed to provide the fluidising steam, withthe bulk of the steam condensing in a heat exchanger immersed in thebed. As the evaporated moisture is recovered in liquid form, the processoffers major efficiency advantages over conventional evaporative dryingsystems, while the steam fluidising medium provides major safetybenefits. This idea has not been commercialised due to the poor heattransfer between the steam tubes and the coal.

The Danish “Viking Gasifier” is a biomass version of a small scaleintegrated dryer gasifier. This gasifier is a two-step gasification unitwith a screw conveyer acting as an externally heated pyrolyser and anauto thermal char gasifier. The biomass is fed directly in to thehorizontal screw pyrolyser who has externally heated walls off about600° C. The fuel is dried and pyrolysed during 30-60 minutes leaving drychar and volatiles as result.

Gas released from the pyrolysis enters the oxidation zone where it ismixed with steam and some air to combust a small part of the pyrolysisgas which elevate the temperature to about 1150-1400° C. The hightemperature decomposes almost all tars to simple gases in a fraction ofa second, but some soot is formed. When the char leaves the pyrolyser itfalls down through the oxidation zone to the bottom of the actualgasifier that is a solid bed down-draft gasifier. Steam and chargasification is a highly endothermic reaction so the temperaturedecreases about 100° C. through the bed; from 700 to 600° C. The glowingchar bed also decreases tar content in the produced gas to between 10 to30 mg/Nm³ before it leaves the reactor.

After the gasifier a series of coolers and filters are installed, ablower forces the gas in to a gas engine that is coupled to a generatorto produce electricity. Exhaust gases from the gas engine heats up thewalls inside the pyrolyser by an integrated heat exchanger.

The Viking gasifier has the advantage of low tar content syngas withoutan extra tar cracker step.

The slowest process step of the Viking Gasifier is the indirect dryerleading to the patent suggesting using a superheated steam dryer as apossible pre-step. However when one considers the possible feed stocksof manure and municipal solid waste with their high sulphur, ammonia andhalogen loading a single step dryer loses appeal. In fact it becomesundesirable to return the drier's exhaust to the pyrolyser. Thereforethe Viking gasifier is limited to relatively clean wood waste.

Also the Viking gasifer has low syn-gas CV leading to higher capitalcost per KW at the gas engine and low power output for the plants size.

Accordingly, there is a need for a drying and gasification process forcarbonaceous substances, such as biomass and low rank coal, that hassignificantly lower installation costs whilst still maintaining highenergy conversion and low CO₂ emissions. There is also a need for adrying and gasification process with an ability to cope with a widerange of biomass types and size distribution; including manure, greenleaves and other wastes, in addition to the traditional chipped wood.Furthermore, there is also a need to provide a process of treatingmunicipal and agricultural wastes such as for example from intensivechicken farming which may produce energy from the waste, or whichprovides an end product suitable for general disposal on land.

SUMMARY OF THE INVENTION

According to one aspect the present invention provides a process forproducing syngas from a carbonaceous substance and/or treating acarbonaceous substance, the process including the following steps:

-   -   a) reducing the surface moisture of the carbonaceous substance;    -   b) reducing the inherent moisture of the carbonaceous substance;        and,    -   c) gasifying the carbonaceous substance to produce syngas,        wherein at step a) the carbonaceous substance is directly        contacted with a hot gas at a temperature of between 50° C. to        250° C., and/or the carbonaceous substance is indirectly        contacted with saturated steam at a temperature of between        105° C. and 250° C.

According to one embodiment at step a) the hot gas is a waste gas from acombustion process, or, the hot gas is indirectly heated by waste heatbefore contacting the carbonaceous substance. Preferably, thecarbonaceous substance is at least partially fluidised when contactedwith the hot gas and the temperature of the carbonaceous substance isbetween 25° C. and 100° C. after step a) and before step b) and thesurface moisture content of the carbonaceous substance is reducedcompared to the its surface moisture content prior to step a).

According to one embodiment at step b) the carbonaceous substance isdirectly contacted with a hot gas at a temperature of 100° C. and 300°C., and may, or may not, be indirectly contacted with saturated steam ata temperature of between 150° C. and 250° C. Preferably, the hot gas isa waste gas from a combustion process, or, the hot gas is indirectlyheated by waste heat before contacting the carbonaceous substance.Preferably, the hot gas has a reduced oxygen content from that of airwherein the oxygen content in the hot gas is from between 2% to 15%volume. Preferably, the carbonaceous substance is at least partiallyfluidised when contacted with the hot gas and/or the temperature of thecarbonaceous substance is typically between 80° C. and 150° C. and themoisture content of the carbonaceous substance typically is from 2% to20% wt after step b) and before step c).

According to one form the outlet gas from step a) and/or step b),produced after the hot gas directly contacts the carbonaceous substance,is directed to a water recovery process step to recover the moistureremoved from the carbonaceous substance and/or the outlet gas is treatedto remove potentially hazardous gaseous compounds yielded from thecarbonaceous substance, such as for example ammonia and hydrogencyanide.

According to one embodiment, at process step c) the carbonaceousmaterial is contacted with hot gas at a temperature of between 500° C.and 1000° C. yielding a gas stream of syngas and solid char. Preferably,the carbonaceous material is at least partially heated by low temperateoxidation of the carbonaceous material wherein oxygen is added to thehot gas before being contacted with the carbonaceous material at step c)to control the degree of low temperature oxidation of the carbonaceousmaterial.

According to another embodiment at process step c) the carbonaceousmaterial is contacted with hot gas at a temperature of between 200° C.and 500° C. yielding a gas stream of low CV syngas and a carbonaceoussubstance that is at least partially pyrolised wherein the syngasproduced by the process is combusted to form the hot gas, and/or thewaste heat utilised in step a), step b) and/or step c). Preferably, thepartially pyrolised carbonaceous substance is used as a feed stock for acombustion process such as a for example a boiler for power generation.Alternatively, the partially pyrolised carbonaceous substance isgasified in a further step to produce a higher CV syngas which may beused in a gas engine or gas turbine.

In a preferred embodiment, the high CV syngas is cooled to a temperatureof between 3° C. and 25° C. and excess water separated from the gasstream wherein the excess water separated from the gas stream isrecycled and injected into the gasification steps c) and/or the furthergasification step.

Preferably, the carbonaceous material is chosen from a material, or amixture of materials, that has a relatively high moisture content and/orlow cross over temperature such as for example low rank coal includingbrown and/or sub-bituminous coal, biomass including wood and bagasse,tar sands, municipal solid waste, agricultural or farming waste, animalwaste, human waste or a mixture thereof. Preferably, the carbonaceoussubstance is substantially granular.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will become better understood from the followingdetailed description of preferred but non-limiting embodiments thereof,described in connection with the accompanying figures, wherein:

FIG. 1 includes graphical representations of CATA runs from Macquarieuniversity (V. Strezov, T. Evans and P. Nelson 2007);

FIG. 2 is a graphical representation of the sensitivity of Red GumGasification to Oxygen Content of Fluidising Gas;

FIG. 3 is a process flow diagram in accordance with one embodiment ofthe present invention;

FIG. 4 is a process flow diagram in accordance with another embodimentof the present invention;

FIG. 5 is a process flow diagram of a further embodiment of the presentinvention; and,

FIG. 6 is a process flow diagram of a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION AND EMBODIMENTS THEREOF

As used herein the term “syngas” refers to a gas mixture that includesvarying amounts of carbon monoxide, methane and hydrogen generated bythe gasification of a carbonaceous substance to a gaseous product with aheating value.

As used herein the term “carbonaceous substance” refers to a substancethat is consisting of, containing of, or capable of yielding carbon.

In accordance with one embodiment, the present invention provides anintegrated process for drying and partial gasification of a carbonaceoussubstance including biomass, peat, agricultural and/or municipal waste,low rank coal and tar sands. The process includes the following steps:

-   -   a) reducing the surface moisture of the carbonaceous substance;    -   b) reducing the inherent moisture of the carbonaceous substance;        and,    -   c) gasifying the carbonaceous substance to produce syngas or        boiler fuel.

The process is particularly suited to using carbonaceous substances thathave a high moisture content.

At process step a) the carbonaceous substance is contacted with a hotgas. The hot gas is preferably chosen from air, or a waste gas from acombustion process (eg gas engine exhaust or waste gas) and at atemperature of between 50° C. and 250° C. which has the effect ofdriving off the surface moisture of the carbonaceous substance. At thisstage, it is also preferable that the carbonaceous substance is alsoindirectly contacted with saturated steam at step a, where thetemperature of the saturated steam is between 105° C. to 250° C.

In one preferred form, process step a) can be carried out in a sideblown partially fluidised bed reactor such as that described in WO2007/137330. In this preferred form, the carbonaceous substance moves asa bed through the reactor where it is contacted by hot gas at about 3kPa to 8 kPa being introduced from the side of the reactor whichpartially fluidises the carbonaceous substance removing surface moistureand gently heating up the carbonaceous substance such that the materialexiting step a) is at a temperature of between 25° C. to 100° C. andsome, and preferably most of the surface moisture is removed.

In addition to the hot gas, the carbonaceous substance is indirectlycontacted by saturated steam moving through pipes integral with the bedand that are in contact with the carbonaceous substance moving throughthe fluidised bed. This aids in supplying heat to the hot gas andcarbonaceous substance assisting the removal of surface moisture.

At process step b), the now warm carbonaceous substance exiting step a)is again directly contacted with a hot gas typically in the range ofbetween 150° C. and 250° C. Preferably, the hot gas has a reduced oxygencontent to that of air wherein the oxygen level is between 2% and 15%volume. According to a preferred for the hot gas is a waste gas from acombustion process, and may be chosen from the exhaust gas produced fromcombusting the syngas produced at step c).

Alternatively, or preferably in addition to, the carbonaceous substanceis also indirectly contacted with saturated steam at a temperature of150° C. to 250° C. which assists in reducing the inherent moisturecontent of the carbonaceous substance.

In one preferred form, process step b) can be carried out in a sideblown partially fluidised bed reactor such as that described in WO2007/137330. In this preferred form, the carbonaceous substance moves asa bed through the reactor where it is contacted by hot gas with reducedoxygen content at 150° C. to 250° C. and 3 kPa to 8 kPa being introducedfrom the side of the reactor which partially fluidises the carbonaceoussubstance reducing the inherent moisture and further heating up thecarbonaceous substance such that the material exiting step b) is at atemperature of between 80° C. to 150° C. and at a moisture content of 2%to 20%.

The drying steps of step a) and step b) may be connected, or separated,depending on the desirability of keeping the exhaust gases separate.

The exhaust gases that are given off from both steps a) and b) may betreated to remove their moisture content and recover water taken fromthe carbonaceous substance as well as removing potentially undesirablegaseous products. If a carbonaceous substance including chicken or pigmanure is used, the this product gives off ammonia and hydrogen cyanidewhen heated to temperatures of between 100° C. and 200° C. By removingthe exhaust gas and scrubbing them after process step a) and/or b) theseundesirable products can be removed allowing the disposal of thesubsequent waste in the form of water or particulate matter.

The third gasification process step c) can be done in various waysdepending on the desired gas energy density (CV), tolerance of tarcarryover and reactivity of the carbonaceous material. In one embodimentat step c) the carbonaceous material is contacted with hot waste gas ata temperature not exceeding 450° C. yielding a gas stream of a very lowCV syn-gas and char, and ash. This gasifying waste gas may be temperedwith steam and oxygen to reduce tar carryover or to increase the gas CV.Oxygen may be added to the gas stream achieve the desired operatingtemperature in the gasifier. This gasification step is much lower intemperature than typical gasification steps, and does not completelygasify the carbonaceous material leaving a product that may be used as aboiler fuel. Throughout the example embodiments and diagrams in thispatent specification this step is referred to as CO₂ Removal dryerand/or gasification step as this step removes a portion of the oxygen inthe carbonaceous feed in the form of evolved CO₂.

The exhaust gas or syngas produced is burnt in after burners and use asa low grade waste heat. Typically, about half of the evolved CO₂ and 25%of the CO are produced by 350° C. for biomass and 400° C. for low rankcoal. Therefore we have called this step CO₂ Removal dryer and it can beused to produce a higher CV syn-gas (by not mixing the CO₂ with a chargasifier syn-gas) or a hot char which performs like a high rank coal ina furnace or boiler. This embodiment is more suited to gas use in a gasturbine or as a pre-process to a traditional open cycle power plant

In a second embodiment of step c) which has higher temperature where thegas used in other subsequent steps and the feed from step B is alsohotter. We call this embodiment gasifier as the unit is operated morelike a conventional gasifier. This approach leads to hotter charentering any subsequent steps. For biomass the char entrance temperatureneeds to be hotter than 400° C. for low tar yield operations in thesubsequent step. The corresponding temperature for lignite is hotterthan 500° C. This method reduces the tar carryover and approaches thegas cleanliness of a traditional down draft gasifier for any subsequentstep. This embodiment is more suited to gas engines or otherapplications that are excessively tar sensitive. It is also thepreferred embodiment where there is co-fuelling eg biomass with coal,biomass with municipal solid waste, municipal or agricultural solidwaste and coal, municipal or agricultural solid waste and biomass. Thisis due to the vastly different temperatures of gasification and reactionrates between the fuels and the ability of the split gasification toovercome these differences.

To economise the process, and to take advantage of bio-char highreactivity, step c) can be done with waste gas from a combustion process(eg gas engine exhaust) or even the waste gas from Step B which has avery high moisture content. The fluidising gas is heated and itstemperature is typically between 200° C. and 600° C. depending on oxygencontent. Bio chars and brown coal typically have a cross overtemperature between 120 and 150° C. That is when the biomass is hotterthan 120° C. and the oxygen content of the fluidising gas is between 4%to 5% oxygen the rate of gasification starts to be fast enough to beused commercially. Refer to FIG. 2. At such a low temperature the heatloss to the fluidising gas is minimal. This makes the thermal efficiencyof gasification in step c) very good, leading to less bio-mass beingneeded to run a certain size power station. The disadvantage is that thegasifier needs to be slightly larger to allow for the increased requiredresidence time.

By separating out the various steps associated with drying andgasification and performing these at relatively low temperatures and gaspressures compared to existing gasification arrangements, the presentinvention is able to be undertaken in a low cost installation that canbe situated close to the source of a carbonaceous substance due to theeconomies of scale. This substantially reduces transportation costsincreasing the viability of using various carbonaceous substances asbiofuels.

The process of the present invention may be suitable for a wide range ofbiomasses and other irregular fuels of varying size and distribution asthe process allows the use of fluidised beds for the drying andgasification process steps. In addition, by separating out the surfacemoisture drying step a) and inherent moisture drying step b), theprocess provides increased flexibility to separate non-desirable gasesthat can be produced during these steps when using various types ofbiomass feeds such as municipal waste, and agricultural wastes includingmanure.

The process of the present invention also provides increased flexibilityto cater for the large changes in density associated with drying andde-volatilisation and also provides reduced sensitivity to contaminationof feedstock eg dead chickens included in chicken manure from batteryfarm operations.

There process includes fewer problems associated with tar carry over dueto the use of steam, oxygen as tempering gases (and CO₂ in the recycledgas) as well as the ability to have hotter feed material because of thepreheating in the final stage of drying/partial gasification. In apreferred form the process has the ability to take the tar laden, lowtemperature gasification off gas, and recycle pass this into the hottestgasifier or destroy these compounds in after-burners.

The process of the present invention provides significant economicadvantages due in part to the low cost of construction as the processallows the use of fluidised beds with lower overall height compared totraditional fluidised beds and which are designed for low temperatureand pressure operation. No necessity for super heated steam, althoughsuperheated steam can be use if the steam systems already exist. Inaddition, the concept has modular scalability so, factory built, skidmounted dryer, gasifiers and heat exchanges can be delivered to sitewith minimum site erection.

The process of the present invention also has high efficiency due torecycling surface moisture off gas to the gasifier (after NH₄ and HS₂removal if required) reducing the need to raise steam. In addition, theprocess uses the waste heat in the exhaust gas from the gas engine orafter burners to re-heat the steam to be used in the inherent moisturedryer. Other efficiency advantages are: the use of the waste heat in thesyngas from the char gasifyier to re-heat the water used in the inherentmoisture dryer; using the waste gas post the steam raising in thesurface moisture dryer; using the syngas from the CO₂ Removaldryer/gasifier as the fluidising gas of the second step gasifier; andusing the sensible heat in the exhaust gas from the gas engine topre-heat the exhaust that is the fluidising gas of the biomass gasifier.

It is also possible to have higher power output from gas engines inaccordance with one application of the present invention due to highercalorific value gas. This can be accomplished by oxygen enrichmentfiring of the gasifier which provides that a higher amount of sensibleheat can be recovered and reused in fluidising gas and also provides ahigher moisture content of fluidising gas. In addition, the resultingsyngas can be dried which to recover water from the process.

The process of the present invention is very easy to scale into verysmall units due to no dependence on gas or steam turbines, the modularnature of design, the relative low height of the installation due to theuse of fluidised beds, and the split nature of process steps. Each ofthese characteristics provides that the present invention can be usedfor a simple rural based design for hot or dry climates as well asvarious other applications as in particular there is no necessity for awater treatment plant or cooling towers.

The process according to the present invention has many environmentaladvantages as it allows wet biomass to turned in to electricity moreefficiently. This means that a smaller amount of biomass is neededbefore there is sufficient biomass to build a small power station.Therefore, more biomass power stations can be economically built.Approximately 1MW_(e) hour of biomass energy will save the environment1000 kg of CO₂ emission.

The process of the present invention allows the ability to have moredistributed biomass power generation at distance from the major coalburning power stations which thereby reduce line loses. For example 115kW is needed to supply 100 kW of power at a distance of 300 km from thepower station.

-   -   According to one embodiment of the present invention allows wet        coal to be turned into electricity more efficiently. Therefore,        less coal is burned to produce the same amount of electricity.        For brown coal this approximately 400 kg of CO₂ emission, is        reduced per 1MW_(e) generated.    -   In addition, the process of the present invention is not highly        water dependent as it is not reliant on external water for        cooling towers and to make for steam losses. This is achieved in        three ways:        -   Using gas engines not open cycle steam turbines; and        -   Using the heat in the waste gas to cool and dehumidify the            syn-gas to increase gas engine performance and the bag house            gas when the water need is high enough.        -   Reusing the recovered water as the processes make up water.

For example; by incorporating the process of the present invention inVictoria's Wimmera region, a 10 MW, biomass power station would save92000 t of CO₂ emission per year as well as create rural jobs and notuse any external water.

Syngas produced by the process of the present invention could be used inthe smaller plants for power generation, with a gas engine where theengine is also the hot gas generator. Syngas could also be used forcombined cycle power generation on larger plants. Syngas has also beenused for manufacturing transport fuel using the Fisher Trope process totypically make ethanol or diesel.

Biomass is not a homogenous substance. Reference is made to FIG. 1. Theoperating condition to achieve best gasification performance has to bevaried for each type of biomass.

“ . . . . The release of gaseous compounds during biomass wastepyrolysis consisted of four main stages. During the first stage,strongly bonded hydrated compounds are released to form a condensablewater fraction . . . . The second stage involves evolution of CO and CO₂compounds with the peak evolution rate detected at around 370° C.Thirdly, hydrocarbons are released at higher temperatures as products ofsecondary cracking of the tar fraction, with methane being thepredominant compound. The fourth stage is the release of hydrogen attemperatures above 600° C.” (V Strezov, L Strezov and J Herbertson,2005)

The fluidising gas composition has a strong impact on the rate ofgasification, as shown in FIG. 2, as does the degree of carbonisation.The rate of gasification of char is similar to wood at one fifth theoxygen content in the fluidising gas. That is, wood gasification rate at4% oxygen is similar to wood char gasification rate at 21% oxygen.Splitting the dryer/gasifier into a number of steps allows completecontrol over the various reaction rates. It also caters for the verydifferent density and size of particles at the various stages betweenfeed and final char burn out. This makes selective use of oxygenenrichment move attractive. This is not possible with the HRL design.The splitting of the dryer also makes the workable but un-economic SteamFluidised Bed Drying economically attractive due to increased heattransfer rates and it makes such a system scalable down to biomass size.

By splitting the dryer into many vessels and only the first (andpossible second) vessel has the steam tubes then the heat transfer ratecan be improved as the biomass entering the dryer is cooler compared tothe steam. Likewise, if the biomass can be made to move in plug flowrelative to the steam pipe, compared to mixed tank flow, then the heattransfer rate can again be improved. Splitting the dryer, so that thesteam tubes only contacts the fresh biomass, ensures the highest rate ofheat transfer for the lowest possible steam pressure. In this way steamat a pressure as low as 60 to 80 kPa (gauge) can be put to work heatingbiomass at 25 to 100° C. Also, by using incline side blown fluidised bed(International Patent Application No. PCT/AU2007/000718), the biomassapproaches “plug flow” rather than the traditional “mixed tank flow”found on most fluidised bed systems. This plug flow also helps makingthe low pressure steam useful.

One of the commonly used criteria for gasification is carbon conversion.This is defined as:

Carbon conversion=1−(carbon in char)÷(carbon in solid feed stock)

To achieve greater carbon conversion, process designers have selectedhigher temperatures and pressures. As a result, the capital costs oftheir plants have increased. However, in rural communities, the ash fromthe process and activated carbon mixed with this ash make an excellentfertiliser and soil conditioner. Therefore, for biomass applications, itis not that important to have a very high carbon conversion. This leadsto the potential of the present invention which enables the constructionof biomass processes at lower temperature and pressure compared totradition gasifiers used for coal. This leads to greatergeo-sequestration of carbon as the carbon in the ash is returned to theground. In addition, the higher temperature portion of the process arevery much smaller if it is staged.

The process of the present invention may be modified for peat, low rankcoal and tar sands if the feed stock was sufficiently reactive. Testwork with Victorian lignite has shown that this coal is reactive enoughto gasify at low temperature and pressure.

The present invention will now be further described in connection withthe following examples of applications of the process of the presentinvention:

Example 1

Example 1 is for a plant to produce a hot bio-char as a feed to aconventional coal based power station. Referring to FIG. 3, there isshown a process flow diagram in accordance with one embodiment of thepresent invention. As can be seen, the carbonaceous material proceedsthrough three process steps which ultimately results in the productionof low grade syn-gas.

In the first process step a) the carbonaceous substance enters a surfacemoisture dryer where the carbonaceous substance is contacted with a hotgas at a temperature of between 115° C. and 400° C. According to thisembodiment, the hot gas is a waste gas from the power station. Inaddition to the direct contact with the hot gas, the surface moisturedryer also indirectly contacts the carbonaceous substance with steamflowing through steam tubes within the dryer. The action of the steamindirectly contacting the carbonaceous substance slowly heats thesubstance to between 100° C. and 140° C. resulting in the steamcondensing and being removed from the surface moisture dryer as liquidwater. Typically, the steam entering the surface moisture dryer at stepa) has a temperature of between 105° C. to 175° C.

The result of the first process step a) involving the surface moisturedryer provides an exit stream of carbonaceous substance with reducedsurface moisture content and at a temperature of between 100° C. and140° C. In addition, the hot gas exiting the surface moisture dryer atstep a) includes the moisture removed as steam from the carbonaceoussubstance together with fine particles carried through with the gasstream. A portion of this warm gas may be reheated by a hot gasgenerator and then used as the feed gas for the gasification step c).

This first process step a) could be carried out using a variety ofdevices such as a steam heated rotary kiln or hearth but is morepreferentially carried out using a horizontal, differentially fluidisedbed surface moisture dryer.

A rotary lock device such as a screw conveyor or rotary valve separatesthe surface moisture drier at process step a) and provides the heated upcarbonaceous material to the second process step b) which in thisembodiment takes the form of an inherent moisture drier.

In the second process step b), the carbonaceous substance is contactedwith low-pressure superheated steam. The steam temperature is typicallybetween 400° C. and 850° C., and as a result this process provides acarbonaceous substance with substantially reduced inherent moisture.Typically, the temperature of the carbonaceous substance is raisedduring this process step to between 120° C. to 220° C. The secondprocess step b) could be executed in a variety of devices such as aconventional rotary kiln or tray drier but is more preferentially donein a horizontal, differentially fluidised bed.

Preferably, the resultant gas stream of steam from the inherent moisturedrier of process step b) is recompressed. Some of this recompressedsteam is recycled into the steam superheater that feeds the inherentmoisture drier and the balance is fed into the first process step a)steam tubes of the surface moisture dryer. In the instance the feedcarbonaceous substance has a very high moisture content the steambalance is in slight deficit, and feed substances with lower moisturecontent the steam balance is slightly positive.

In this embodiment, the process heat for the steam superheater and thehot gas entering the surface moisture dryer at step a) is provided by astand-alone hot gas generator (HGG). This HGG preferentially burns wetcarbonaceous material so that the waste gas has a high water content.Preferably this wet carbonaceous material is the fine carbonaceousmaterial blown out of the surface moisture drier.

The heat exchanger depicted in FIG. 3 cools the hot gas entering step a)to between 115° C. to 400° C. and separately heats the super heatedsteam for entry into step b) and the hot gas for use in step c) up to400° C. to 850° C.

A rotary lock device such as a screw conveyor or rotary value transfersthe carbonaceous substance now with very little surface moisture orinherent moisture from the inherent moisture drier to the gasifier atprocess step c).

At process step c) the gasification of the carbonaceous material isprovided by contacting the carbonaceous material with a stream of hotgas. Preferably the hot gas is at a temperature of between 700° C. to750° C. The hot gas is chosen from one of, or a mixture of, oxygen,steam and/or air. More preferably to get maximum efficiency the gassteam is a mixture of oxygen and heated off gas that exists from thesurface moisture drier at process step a).

Due to the very low temperature and pressure of the gasifier the charyield is relatively high. The bio-char is used like a high rank coalwithin the power station.

This third step could be executed in a variety of devices such as aconventional rotary kiln or tray drier but is more preferentially donein a horizontal, differentially fluidised bed.

It should be understood that various changes, substitutions, andalterations can be made herein by one ordinarily skilled in the artwithout departing from the spirit or scope of the present invention. Forexample FIG. 4, show a different arrangement of heat exchangers and FIG.4 does not use super heated steam, but FIGS. 3 and 4 both use the threesteps to convert a wet carbonaceous material into a char.

Example 2

In the following example, fresh leaves and branches have beenmechanically pick up from the ground. The average collection rate is1.25 tonne per hour over the entire year. This biomass material will bebrought to the small central plant where the timber is chipped. Thepower plant is within 25 km of the farm based sustainable timberproduction. The fuel composition is:

Total Moisture 50.00% w.b. Ash 10.80% d.b. Carbon 46.10% d.b. Hydrogen5.90% d.b. Nitrogen 0.60% d.b. Sulfur 0.20% d.b. Oxygen 36.40% d.b.Specific Energy 18.10 MJ/kg d.b (gross)

This amount of biomass, at its stated composition and if it was used inan open cycle power plant would typically produce about 0.46 MW,(gross). This represents an overall conversion efficiency of 15%. Thisis below economic size for an open cycle power plant.

However, if this same biomass was used as a feed source for a plant asoutlined in the process diagram FIG. 5, this resource could be usedquite effectively. In FIG. 5 the wood waste proceeds through threeprocess steps a), b) and c) which ultimately results in the productionof syngas at step d). At step d) the hot char which exits the partialgasification step c) is processed in a conventional gasifier at step d)and the sensible heat from that gasifier is used to heat steam which isthen used in the removal of surface moisture and inherent moisture atsteps a) and b). The syngas is used to drive a gas engine or gas turbineat step e) and the exhaust gases can then be used for direct contactingthe biomass in steps a) or b) or for providing waste heat to indirectlyheat hot gas.

In the first process step a) the wood waste enters a surface moisturedryer in the form of a fluidised bed reactor with indirect steamheating, which is 3 m long, 0.5 m wide and has a steam tube area of 15m². In this surface moisture dryer the wet wood waste is contacted with500 kg/h of hot air at a temperature of 200° C. which partiallyfluidises the wet wood waste within the reactor. This hot air comes froma heat exchanger used to cool the waste gas from the gas engine/gasturbine at step e). In addition to the direct contact with the hot gas,the surface moisture dryer also indirectly contacts the wood waste withsaturated steam flowing through steam tubes within the dryer. The steamis at 150° C. The action of the steam indirectly contacting thecarbonaceous substance slowly heats the wood waste to 95° C., resultingin the 380 kg/h of steam condensing and being removed from the surfacemoisture dryer as liquid water. The surface moisture dryer doesapproximately 150 kW of drying work. About 72% of the heating and dryingwork is done by the condensing steam. The wood waste leaves this dryerat approximately 39% moisture.

In this embodiment step a) and step b) are undertaken the same vesselwhich totals 6 m in length but the duct work of the fluidised beddelivers hotter and lower oxygen gas to the inherent moisture portion ofthe vessel compared to the surface moisture portion.

In the second process Step b, the wood waste is contacted with 500 kg/hof waste gas at 250° C. The wood waste's temperature increases from 95°C. to 135° C. and its moisture drops from 39% to 16%. The inherentmoisture portion of the vessel is 3 m long and 0.5 m wide. The steamtemperature is 150° C. and steam tube area is 15 m².

The resultant exhaust from step a) and step b) is de-dusted and thensent to a bag house after which the gas can be sent to a clean waterrecovery unit to remove the moisture taken from the wood waste.

A screw conveyor transfers the wood waste at 135° C. and 16% moisturefrom step b) to the gasifier at process step C. At this point thegasification of the wood waste is provided by contacting the wood wastewith the high humidity waste gas from the surface moisture dryer whichhas been enriched with oxygen from a PSA oxygen plant. The waste gas hasbeen enriched to 5.0% oxygen by volume and heated to 400° C. in a heatexchanger cooling the waste gas product syngas. The wood is rapidlydried and partially gasified at step c) and some tars and oils are alsoproduced as well as CO₂ and CO.

The wood char leaves the biomass gasifier at step c) at about 450° C.and enters the char gasifier step d) via a screw conveyor. The off gasfrom Step c) is partially oxidised to convert the sulphur compounds intosulphates which are trapped in the matrix of hot dolomite in a sulphurscrubber prior to entering step d). With the removal of sulphurcompounds the gas can be past over methantion catalyst to lower the H₂and CO content and raise the CH₄ content. This reduces potentialknocking at the gas engine at step e). This gas, which is now at about700° C., enters step d).

The hot fluidising gas in step d) has high humidity which helps ensuregood tar and oil conversion to gas. Step d) is typically held at 800 to900° C. with the staged addition of oxygen. Liquid water is added to thechar ash exiting step d) and the steam from this cooling step rises intothe gasifier increasing the humidity.

The syngas produced by step d) is cooled through two heat exchangersrunning in parallel. The syngas is cooled further in a heat collectionheat exchanger for the absorption chiller.

The chiller is used to cool the water in the liquid ring compressorwhich scrubs the gas and removes particulates and raises it pressure to100 KPa(g). Approximately 950 m³/h of 8.5 MJ/m³ syngas is produced whichtranslates to approximately 1.0 MWe of power (gross) at Step d) orapproximately 0.75 MWe (net). The overall conversion efficiency, thermalto electrical, is 26%, even at this micro scale. This is similar to opencycle at the plus 15MW_(e) scale.

In this embodiment, four identical heat exchanges support the process.

-   -   Two Steam Heater: cools the syngas from the char gasifier and        heats the steam for the inherent moisture drier.    -   Fluidising Gas Pre Heater: cools the engine exhaust and heats        the waste gas from the surface moisture dryer.    -   Exhaust Cooler: cools the gas engine exhaust and heats the steam        for the inherent moisture drier.

Whilst other gasifiers have a higher carbon conversation efficiency thiscomes at the price associated with higher temperature and pressure whichis an unnecessary cost for most rural based biomasses and wastes. Thisexample plant would save approximately 6000 to 6700 tonne of CO₂emission each year and create rural jobs.

Although several embodiments have been described in detail otherembodiments of the integrated dryer and gasifier are possible. Thisprocess could be modified for ethanol production by burning the low CVoff gas from the Step c in an after-burner and using steam in the chargasifier to improve the H₂ to CO ratio before the Fisher Trope process.Likewise, gas and steam turbine could replace the gas engines in biggerplants to improve the efficiency and reduce maintenance. FIG. 6illustrates how to apply the concept to Municipal Solid Waste (MSW),where the undesirable metals, halogens and other gases are evolvedbetween 140° C. and 250° C. Hence the scrubbing systems are betweensteps b) and c) in this example. If there was significant manure loadthen one would add a scrubber between steps a) and b) that is between50° C. and 100° C.

It should be understood that various changes, substitutions, andalterations can be made herein by one ordinarily skilled in the artwithout departing from the spirit or scope of the present invention.

1. A process for producing syngas from a carbonaceous substance and/ortreating a carbonaceous substance, the process including the followingsteps: a) reducing the surface moisture of the carbonaceous substance;b) reducing the inherent moisture of the carbonaceous substance; and, c)gasifying the carbonaceous substance to produce syngas, wherein at stepa) the carbonaceous substance is directly contacted with a hot gas at atemperature of between 50° C. to 250° C., and/or the carbonaceoussubstance is indirectly contacted with saturated steam at a temperatureof between 105° C. and 250° C., and wherein the temperature of thecarbonaceous substance after step b) and before step c) is between 80°C. and 220° C.
 2. A process according to claim 1 wherein at step a) thehot gas is a waste gas from a combustion process, or, the hot gas isindirectly heated by waste heat before contacting the carbonaceoussubstance.
 3. A process according to claim 1 wherein at step a) thecarbonaceous substance is at least partially fluidised when contactedwith the hot gas.
 4. A process according to claim 1 wherein thetemperature of the carbonaceous substance is between 25° C. and 100° C.after step a) and before step b) and some, or most of the surfacemoisture of the carbonaceous substance is removed.
 5. A processaccording to any claim 1 wherein at step b) the carbonaceous substanceis directly contacted with a hot gas at a temperature of 100° C. and300° C., and/or the carbonaceous substance is indirectly contacted withsaturated steam at a temperature of between 150° C. and 250° C.
 6. Aprocess according to claim 5 wherein at step b) the hot gas is a wastegas from a combustion process, or, the hot gas is indirectly heated bywaste heat before contacting the carbonaceous substance.
 7. A processaccording to claim 6 wherein at step b) the hot gas has a reduced oxygencontent from that of air wherein the oxygen content in the hot gas isfrom between 2% to 15% volume.
 8. A process according to claim 5 whereinat step b) the carbonaceous substance is at least partially fluidisedwhen contacted with the hot gas.
 9. A process according to claim 5wherein the temperature of the carbonaceous substance is between 80° C.and 150° C. and the moisture content of the carbonaceous substance isfrom 2% to 20% wt after step b) and before step c).
 10. A processaccording to claim 5 wherein an outlet gas from step a) and/or step b),produced after the hot gas directly contacts the carbonaceous substance,is directed to a water recovery process step to recover the moistureremoved from the carbonaceous substance.
 11. A process according toclaim 5 wherein an outlet gas from step a) and/or step b), producedafter the hot gas directly contacts the carbonaceous substance, istreated to remove potentially hazardous gaseous compounds yielded fromthe carbonaceous substance, such as for example ammonia and hydrogencyanide.
 12. A process according to claim 1 wherein process steps a) andb) are performed in the same vessel such as for example a partiallyfluidised bed reactor.
 13. A process according to claim 1 wherein atprocess step c) the carbonaceous material is contacted with hot gas at atemperature of between 500° C. and 1000° C. yielding a gas stream ofsyngas and solid char.
 14. A process according to claim 13 wherein atprocess step c) the carbonaceous material is at least partially heatedby low temperate oxidation of the carbonaceous material.
 15. A processaccording to claim 14 wherein oxygen is added to the hot gas beforebeing contacted with the carbonaceous material at step c) to control thedegree of low temperature oxidation of the carbonaceous material.
 16. Aprocess according to claim 1 wherein at process step c) the carbonaceousmaterial is contacted with hot gas at a temperature of between 200° C.and 500° C. yielding a gas stream of low CV syngas and a carbonaceoussubstance that is at least partially pyrolised.
 17. A process accordingto claim 16 wherein at process step c) the carbonaceous material is atleast partially heated by low temperature oxidation of the carbonaceousmaterial.
 18. A process according to claim 17 wherein oxygen is added tothe hot gas before being contacted with the carbonaceous material atstep c) to control the degree of low temperature oxidation of thecarbonaceous material.
 19. A process according to claim 1 wherein thesyngas produced by the process is combusted to form the hot gas, and/orthe waste heat utilised in step a), step b) and/or step c).
 20. Aprocess according to claim 16 wherein the partially pyrolisedcarbonaceous substance is used as a feed stock for a combustion process.21. A process according to claim 16 wherein the partially pyrolisedcarbonaceous substance is gasified in a further step to produce a higherCV syngas which may be used in a gas engine or gas turbine.
 22. Aprocess according to claim 21 wherein the high CV syngas is cooled to atemperature of between 3° C. and 25° C. and excess water separated fromthe gas stream.
 23. A process according to claim 22 wherein the excesswater separated from the gas stream is recycled and injected into thegasification steps c) and/or the further gasification step.
 24. Aprocess according to claim 1 wherein the carbonaceous material is chosenfrom a material, or a mixture of materials, that has a relatively highmoisture content and/or low cross over temperature.
 25. A processaccording to claim 1 wherein the carbonaceous material is a low rankcoal including brown and/or sub-bituminous coal, biomass including woodand bagasse, tar sands, municipal solid waste, agricultural or farmingwaste, animal waste, human waste or a mixture thereof.
 26. A processaccording to claim 1 wherein the carbonaceous substance is substantiallygranular.
 27. A process according to claim 20 wherein the partiallypyrolised carbonaceous substance is used as a feed stock for a boilerfor power generation.